1. Field of the Invention
This invention relates to a microwave frequency discriminator and more particularly to a discriminator comprising a dual-gate field effect transistor including biasing circuits for obtaining electronically predetermined transistor input and output impedance conditions.
2. Description of the Prior Art
Techniques and devices for the rapid and accurate determination of an unknown signal frequency are of significant interest in modern communication systems, in particular for electronic counter measure (ECM) systems operating at microwave frequencies. Microwave frequency discriminators capable of converting incoming unknown frequencies into voltages for processing are often used in ECM systems. A microwave frequency discriminator may be defined as a circuit that provides an output voltage which is a predetermined function of, and usually proportional to, the frequency of an incoming signal. The discriminator voltage output versus frequency response, commonly termed the "discriminator characteristic," is the response in which the output voltage varies nearly linearly with respect to frequency over a predetermined frequency bandwidth. The bandwidth is generally determined by the slope, linearity, and resolution of the discriminator and is the frequency range over which the descriminator provides an unambiguous voltage output which is related to the input frequency.
A typical broadband microwave discriminator often utilized in modern systems comprises passive elements such as transmission lines or an arrangement of lumped elements to vary the power level of the input signal applied to a detector diode as a function of frequency. Such a disciminator formed of various passive components, generally has, disadvantageously, a large number of connections between the components. These interconnections often produce impedance mismatches resulting in undesirable inflection points in the relation between input frequency and output voltage such that a particular voltage can occur at several frequencies. Such distortions in the discriminator characteristic limit the broadband resolution of the discriminator minimizing thereby the accuracy of the system.
To overcome such discriminator deficiencies, recent advances in the art have resulted in the minimization or elimination of the passive elements and the utilization instead of active elements, such as field effect transistors, to achieve the discriminator characteristic. As disclosed in U.S. Pat. Nos. 4,053,841, entitled, "Improved Microwave Frequency Discriminator Comprising An FET Amplifier," by A. Rosen et al., and 4,053,842, entitled, "Microwave Frequency Discriminator Comprising An FET Amplifier," by Z. Turski, FETs are arranged to produce an output voltage that varies substantially linearly with the frequency of an input RF signal throughout a certain frequency bandwidth. The impedance conditions for obtaining the discriminator characteristic with the FET are achieved in the aforementioned U.S. Pat. No. 4,053,842 by input and output shaping networks, and in U.S. Pat. No. 4,053,841 by an input shaping network and an output biasing circuit. The shaping networks in each patent are typically arranged in microstrip form with the geometric shape of the network determining the impedance condition. The configuration for providing a desirable impedance is selected for a specific range of discriminator operating conditions. The configuration of the shaping networks for a given set of discriminator parameters is obtained by the use of computer iteration techniques to determine the particular geometric pattern. Fine tuning of the discriminator characteristic is achieved by manually varying the location of tuning stubs on the microstrip circuit.
Although an improvement over the passive element type discriminator, the FET discriminator utilizing particularly configured shaping networks disadvantageously requires a selection process for determining the network geometry and thereby its impedance as well as the manual process of tuning by the manipulation of tuning elements or stubs. Such processes restrict the discriminator performance to a limited range of operating conditions since, once the configuration of the networks is determined, it is fixed, subject only to minimal adjustments by manual tuning. Manual tuning is problematical in such applications as well, since once the discriminator is in its intended operating environment, lack of accessibility may prohibit any further manual tuning.
It is further known in the microwave art that the problematical manual tuning may be replaced by electronic tuning provided by a FET amplifier including an electronic variable capacitor (varactor), a biasing circuit and a detector. Such a circuit is described in U.S. Pat. No. 4,110,700, entitled "Electronically Tunable Microwave Frequency FET Discriminator," by A. Rosen et al. There remains a need in the microwave art to provide an electronically tunable FET discriminator that operates without the use of an electronic variable capacitor such as a varactor.